An aircraft, such as an airplane, has a multitude of movable elements associated with actuators that are connected to a piloting unit enabling the pilot of the airplane to control the actuators directly or indirectly.
By way of example, the movable elements may be:                wing surfaces or flight surfaces, in particular ailerons, spoilers, and other flaps; and        landing gear elements, in particular hatches, undercarriage legs, or the nose wheel(s).        
Conventionally, the actuators have been hydraulic actuators, and in particular actuators that are connected via solenoid valves to one or more hydraulic circuits under pressure. The use of hydraulic energy nevertheless raises problems of weight and maintenance constraints that are causing airplane manufacturers to minimize the dimensions of the hydraulic circuits as much as possible.
Proposals have therefore been made to associate each actuator with an individual hydraulic circuit that incorporates an electric pump. There are then no longer one or more hydraulic circuits of large size serving all of the actuators of the airplane, but rather a plurality of small-sized hydraulic circuits, each connected to a single actuator or to a very small number of actuators that need to be operated simultaneously. That architecture has greatly reduced constraints associated with using hydraulics on airplanes.
Nevertheless, historically the Air Transport Association (ATA) has defined a functional coding system for classifying the various systems and equipments present on an airplane depending on the functions they perform in an airplane. By extension, each of those classes is associated with constraints on design and on failure modes to be accommodated so that a piece of equipment that belongs to one particular ATA class is not considered as being usable in some other ATA class.